The present invention relates to a voltage converter (buck converter).
Such a voltage converter is disclosed, for example, in B. Murari, F. Bertotti, G. A. Vignola: xe2x80x9cSmart Power ICsxe2x80x94Technologies and Applicationsxe2x80x9d, Springer Verlag, Berlin, 1996, page 287, or in U. Tietze, Ch. Schenk: xe2x80x9cHalbleiterschaltungstechnikxe2x80x9d [Semiconductor Circuitry], 9th Edition, Springer Verlag, Berlin, 1991, page 564. The construction and the method of operation of such a voltage converter according to the prior art are explained below with reference to FIG. 1.
The task of a voltage converter is to convert a DC voltage V1 into a lower DC voltage V2 for supplying a load RL. To that end, in the prior art voltage converter, a series circuit including a switch S, a coil L, and a capacitor C is connected in parallel with the DC voltage source V1, the load being connected in parallel with the capacitor C. A diode DI is connected in parallel with the series circuit including the coil and the capacitance C. If the switch S is closed, a current flows from the voltage source V1 through the coil L to the capacitance C and through the load RL and the current through the capacitance rises continuously. The DC voltage V1 is present across the diode, the diode DI being in the off state with the switch S closed. If the switch S is subsequently opened, the voltage present across the coil L reverses, the current through the coil L maintaining its direction and beginning to decrease. The reversal of the voltage across the coil has the effect that the potential at the node that is common to the switch and the coil decreases. The diode DI is, thereby, turned on and accepts the current flowing from the coil L to the capacitor C and through the load RL. The series circuit including the coil L and the capacitor C acts as a low-pass filter and converts the voltage V1, which is applied to the series circuit in a clocked manner by the switch S, into a continuous output voltage V2, which is lower than the input voltage V1. The value of the output voltage V2 is adjustable by way of the frequency with which the switch is switched on and off and by way of the time duration for which the switch S is respectively open and closed.
What is problematic, in particular, in the case of very high switching frequencies, is that after the opening of the switch S, when the diode DI is in the on state, charge is stored in the pn junction of the diode DI. This stored charge has the effect that even after the closing of the switch S, when the diode DI is supposed to be in the off state, the diode DI is still briefly in the on state until the stored charge has flowed away. The storage leads to switching losses that increase as the switching frequency rises. Moreover, with the diode DI in the on state, the losses brought about at the diode DI are undesirable.
To reduce these losses, the prior art uses, instead of the diode, a field-effect transistor, in particular, a MOSFET (Metal Oxide Semiconductor Field-Effect Transistor), which, in a manner driven by a drive circuit, is intended to be in the on state whenever the switch is in the off state. The MOSFETs used for such a purpose have an integrated freewheeling diode that is connected in parallel with the drain-source path of the MOSFET and whose connections correspond to the connections of the diode DI according to FIG. 1. This freewheeling diode is formed by virtue of the fact that, in conventional MOSFETs, the source zone and the body zone are short-circuited to obtain a high dielectric strength of the FET. Such FETs are in the off state only with application of a forward voltage in the drain-source direction (forward direction), if no drive voltage is present between gate and source, the forward voltage being a positive voltage in the case of n-channel FETs and a negative voltage in the case of p-channel FETs in the drain-source direction. The dielectric strength may have a value of up to a few hundred volts in the case of power FETs. With application of a voltage in the reverse direction, i.e., with application of a negative drain-source voltage in the case of n-channel FETs and a positive drain-source voltage in the case of p-channel FETs, the conventional FETs are already in the on state when the threshold voltage of the freewheeling diode is reached. Such an effect is desired when conventional FETs are used as a replacement for the diode in voltage converters.
To avoid shunt currents, that is to say, currents that flow away through the switch and directly through the FET, the switch and the FET are not permitted to be in the on state simultaneously. Shortly after the opening of the switch, when the FET is supposed to be in the on state but is not yet fully in the on state, the integrated freewheeling diode of the FET accepts the current from the coil until the FET is fully in the on state. The losses incurred at the FET that is fully in the on state are lower than when using a diode in accordance with FIG. 1 as freewheeling element.
However, in the case of such voltage converters, too, a charge is stored in the freewheeling diode of the FET, the charge having the effect that even after the FET has turned off, the freewheeling diode is still briefly in the on state until the stored charge has flowed away. The affect leads to switching losses that may be considerable, in particular, at high switching frequencies.
It is accordingly an object of the invention to provide a voltage converter that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that reduces the switching losses compared with conventional voltage converters.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a voltage converter, including a pair of input terminals for receiving an input voltage, a series circuit connected in parallel with the pair of input terminals and having a first switch and a low-pass filter having output terminals to be connected to a load, a freewheeling circuit connected in parallel with the low-pass filter, the freewheeling circuit having a second switch, the second switch having a first load path terminal, a second load path terminal, a load path formed between the second load path terminal and the first load path terminal, and a control terminal, and the second switch being a MOS transistor having one of a floating body zone and a source zone, a non-reactive resistor, and a body zone connected to the source zone through the non-reactive resistor.
According to the invention, the freewheeling circuit has a second voltage-controlled switch having a control terminal and a load path formed between a first and second load path terminal, the second switch being configured as a MOSFET whose body zone is formed in a floating fashion, that is to say, is not connected to a defined potential, or is connected to the source zone through a non-reactive resistor. The gate terminal of such a MOSFET forms the control terminal of the second switch, the source and drain terminals form the first and second load path terminals, and the drain-source path of the MOSFET forms the load path of the second switch.
In the case of the MOS transistor used as freewheeling element in the voltage converter according to the invention, the floating configuration of the body zone means that there is no short circuit present between the body zone and the source zone. Consequently, the MOS transistor forming the second switch is in the off state not only with application of a voltage in the forward direction, that is to say, in the case of a positive drain-source voltage in the case of an n-conducting MOSFET and a negative drain-source voltage in the case of a p-conducting MOSFET, but also with application of a voltage in the reverse direction, that is to say, in the case of a negative drain-source voltage in the case of an n-conducting MOSFET and a positive drain-source voltage in the case of a p-conducting MOSFET, if no gate-source voltage is present in each case. Such transistors are also referred to as reverse-blocking FETs (RB-FETs). The blocking voltage of such MOSFETs in the reverse direction (source-drain direction) is usually smaller than the blocking voltage in the forward direction (drain-source direction). The blocking voltage in the source-drain direction lies at least in the region of a few volts and is higher than the threshold voltage of an integrated freewheeling diode in conventional MOSFETs.
When such a MOSFET is used as a second switch, after the opening of the first switch, when the MOSFET is turned on, charge that might lead to switching losses when the MOSFET is subsequently turned off is not stored in the MOSFET.
In accordance with another feature of the invention, there is provided, in the case of a reverse-blocking FET used as a second switch and having a body zone disposed in a floating fashion, for connecting a rectifier element between its first load path terminal, that is to say, the source terminal, and its control terminal, that is to say, the gate terminal. If the first switch is opened in the voltage converter according to the invention, then the voltage present across the load path of the reverse-blocking FET changes its sign and the reverse-blocking FET is turned on in a manner driven by the rectifier element. Because, in the reverse-blocking FET, no freewheeling diode with pn junction is connected directly between the first and second load path terminals, charge storage effects, which might lead to switching losses when the second switch configured as a MOSFET is subsequently turned off, do not occur. The freewheeling diode can be dispensed with in such an embodiment because the reverse-blocking MOSFET used as a second switch is immediately turned on in a manner driven by the rectifier element after the opening of the first switch.
In accordance with a further feature of the invention, a Schottky diode is connected in parallel with the load path of the second switch. If the first switch is opened in a such a voltage converter, then the voltage present across the load path of the second switch changes its sign and the Schottky diode connected in parallel with the load path of the second switch is turned on and accepts the current of an inductance present in the low-pass filter. If the second switch is subsequently turned on in a manner driven by a drive circuit, then the second switch accepts the current, the Schottky diode immediately turning off when the voltage across the load path of the second switch falls below the value of the threshold voltage of the Schottky diode. No charge that may lead to switching losses after the second switch is turned off is stored in the Schottky diode.
In accordance with an added feature of the invention, there is provided a drive circuit that is connected to the control terminal of the second switch to drive the second switch. With the use of a reverse-blocking FET with a rectifier element between the source terminal and the gate terminal, the drive circuit serves for discharging the gate capacitance present in the FETxe2x80x94which gate capacitance is charged through the rectifier element to turn the MOSFET onxe2x80x94through the drive circuit in order, thereby, to turn the MOSFET off.
In accordance with an additional feature of the invention, the low-pass filter has a series circuit with a coil and a capacitor, and the capacitor is connected in parallel with the output terminals.
With the use of a reverse-blocking FET with a Schottky diode connected in parallel and without a rectifier element between source and gate, the drive circuit serves both for driving the FET to turn it on and for driving the FET to turn it off.
In an advantageous manner, the first switch is a transistor, or is configured as a field-effect transistor, in particular, a MOSFET, which is driven by a drive circuit. Provision is expediently made of a drive circuit having two outputs for driving the first and second switches, the drive circuit being configured so as to ensure that only one of the two switches respectively is in the on state. Shunt currents, that is to say, currents that flow back to the voltage source directly through the first and second switches with short-circuiting of the low-pass filter and the load, are, thereby, avoided.
In accordance with yet another feature of the invention, there is provided a drive circuit having an output terminal connected to the first switch for driving the first switch.
In accordance with yet a further feature of the invention, the driving of the first and/or second switch is effected in a manner dependent on an output voltage that can be tapped off at the output terminals. The output voltage can, thus, be readjusted in the event of changes in the load connected to the output terminals. Instead of the output voltage, a signal dependent on a current through the coil of the low-pass configuration can also be fed to the drive circuit.
In accordance with yet an added feature of the invention, there is provided a drive circuit connected to the control terminal of the second switch and driving the second switch dependent upon a switching state of the first switch.
In accordance with yet an additional feature of the invention, there is provided a drive circuit for both of the first and second switches, preferably, connected to the first switch and the second switch.
In accordance with again another feature of the invention, the drive circuit prevents the first switch and the second switch from both being in an on state simultaneously.
In accordance with a concomitant feature of the invention, the second switch is a MOSFET having a body zone and a recombination zone formed in the body zone.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a voltage converter, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.